When water freezes, it expands. This is why ice floats in liquid water. The expansion is quite forceful- this is why ice cracks pavement and leads to potholes.
When you put ice cube trays in the freezer, the surface and edges tend to freeze first, because they are in contact with the cold air. But that shell of ice can stop the water inside from expanding. Sometimes, that water leaks out of a small hole in the top. When that happens, the water exposed to the air freezes, forming a little ring. More water pushes out and freezes, turning that ring into a little stalagmite.
[good video here, including timelapse footage of the spikes growing.](https://youtu.be/5RLQ9WMP2Es?si=SFMR0YzOOModu_ik)
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